Motorcycle engine



FIG. 1 is a right side elevational view of a motorcycle engine showing our new design;

FIG. 2 is a left side elevational view thereof;

FIG. 3 is a rear elevational view thereof;

FIG. 4 is a front elevational view thereof; and

FIG. 5 is a top plan view thereof. 

The ornamental design for a motorcycle engine, as shown. 